1. Field of the Invention
An aspect of the present invention may relate to an antenna system which can be miniaturized with relative ease like an inverted F antenna, and more particularly to an antenna system that is suitably mounted on vehicles.
2. Description of the Related Art
Low-profile inverted F antennas with small dimensions are known. Various antenna systems including improved inverted F antennas have been proposed (see Japanese Unexamined Patent Application Publication No. Hei 10-93332, in particular, pages 2-3 and FIG. 1).
FIG. 11 is a perspective view of a known inverted F antenna. An inverted F antenna 1 composed of a metal plate is mounted on a ground plane 5. The inverted F antenna 1 includes an emission conductive plate 2, a power-supply conductive plate 3, and a short-circuiting conductive plate 4. The emission conductive plate 2 faces the ground plane 5 so that they are parallel to each other. The power-supply conductive plate 3 extends from an edge of the emission conductive plate 2 substantially perpendicular to the emission conductive plate 2 and is connected to a power-supply circuit (not shown). The short-circuiting conductive plate 4 extends from another edge of the emission conductive plate 2 substantially perpendicular to the emission conductive plate 2 and is connected to the ground plane 5. The power-supply conductive plate 3 and the short-circuiting conductive plate 4 are appropriately positioned relative to the emission conductive plate 2 for the best impedance matching. The longitudinal length of the emission conductive plate 2 in FIG. 11 is about one-fourth of the resonance length. The inverted F antenna 1 is easily fabricated by bending a metal plate, leading to reduced manufacturing costs.
Since inverted F antennas and improved inverted F antennas having small dimensions are inexpensively fabricated and also exhibit high gain as described above, they are generally used for vehicle-mounted antennas. Unfortunately, these known inverted F antennas and improved ones do not exhibit sufficiently high gain for vertical polarization, which is required for the vehicle-mounted antennas. With the known inverted F antennas and improved ones, when power is supplied, not only radio waves polarized orthogonal to the emission conductive plate, e.g., vertical polarization, but also radio waves polarized parallel to the emission conductive plate, e.g., horizontal polarization are emitted. Since these antennas have low polarization purity, the gain for radio waves polarized in a particular direction is reduced and thus the antennas cannot achieve desired high gain.